A study of frequency-coded radar signal design was concerned in this paper. A new criterion called ambiguity similitude for optimizing frequency-coded radar signal was proposed，and five different reorder rules based on this novel criterion were given. The simulation results were presented in the end and all results proved that the rules given in this essay are effective and practical，the combined "Range-Doppler" resolution was sharply improved and the "Range-Doppler" coupling was clearly eliminated after the optimizing process，so it can be widely used in frequency-coded radar signal design.